1. Field of the Invention
The present invention pertains to apparatus for blending particulate materials. Specifically, it pertains to apparatus for the mixing and blending of small solid particulate matter utilizing at least to some extent, gravity.
2. Description of the Prior Art
There are many different kinds of gravity blenders which are utilized for the mixing and blending of powder and granular materials or other particulates such as plastic pellets. Examples of such may be seen in U.S. Pat. Nos. 3,647,188; 3,871,626; 4,002,325; 4,128,343; and 4,285,602.
In most of these gravity-type blenders, a vessel is provided for holding the materials to be mixed or blended. Conduits or pipes of some type are normally provided in the vessel for communication with a stream of air or other gas provided by a blower or the like. The materials to be mixed or blended are introduced into the air stream provided by the blower at the lower end of the conduit and conveyed to the upper portion of the vessel through the conduit for recirculation therein. Various means are provided for introducing the materials into the conduit and various types of conduits have been designed. It is generally conceded that a minimum number of three material turnovers is necessary to arrive at an appropriate random mixture. A complete random mixture is essential in some operations such as the plastic manufacturing business to achieve the quality required.
Some of the gravity blenders of the prior art, such as the one shown in U.S. Pat. No. 4,285,602, utilize excessive lengths of conveying conduits and bends. Some also have excessive horizontal sections or support members. Such characteristics require high air and particle velocities and also result in excessive pressure drop in the conveying system. This, of course, requires more expensive equipment and greater energy cost. In addition, particularly with blending of plastic pellets and the like, high velocity causes wear on the particles and excessive build-up of the material on the conduits and bends and supporting members. Blenders which utilize external circulation require a greater number of material turnovers to arrive at a proper mixture.
To avoid some of the above-mentioned disadvantages of external conveying pipe, internal recirculation blenders have been developed such as those cited herein. Most of these internal circulation gravity blenders utilize a central conveying tube into which the material to be blended is metered by annular gaps therearound. In U.S. Pat. No. 3,871,626, the width of the annular gap is, to some degree, variable. However, the internal components of such a blender are relatively complex, resulting in high manufacturing cost. Furthermore, it is difficult to convert existing silos or vessels into blenders utilizing such characteristics.